Desvenlafaxine is chemically known as 4-[2-(dimethylamino)-1-(1-hydroxycyclohexyl)ethyl]phenol. It is a major metabolite of venlafaxine and has been shown to inhibit norepinephrine and serotonin reuptake. Desvenlafaxine, which can also be referred to as O-desmethylvenlafaxine, is represented by the following structural formula:

Desvenlafaxine is first disclosed in U.S. Pat. No. 4,535,186. It describes the process for preparation of desvenlafaxine and its salts with pharmaceutically acceptable organic or inorganic acids like hydrochloric acid, hydrobromic acid, maleic acid, fumaric acid, succinic acid, sulfuric acid, phosphoric acid, tartaric acid, acetic acid, citric acid and oxalic acid.
WO 2009/053840 discloses oxalate, lactate and benzoate salts of desvenlafaxine along with their process of preparation and polymorphic forms. Benzoate salt of desvenlafaxine exhibits a powder X-ray diffraction pattern having peaks at about 5.81, 12.16, 13.22, 14.74, 15.66, 15.90, 16.14, 17.65, 19.40, 19.73, 19.85, 20.46, 22.42, 24.53, 24.81, 25.23, 26.65 and 28.62±0.2 degrees 2θ.
U.S. Pat. No. 6,673,838 discloses the succinate salt of desvenlafaxine and its crystalline forms.
U.S. Pat. No. 7,001,920 discloses the formate salt of desvenlafaxine. Other salts such as maleate, tartarate, aspartate and saccharinate have been described in the literature.
Polymorphism is the occurrence of different crystalline forms of a single compound and it is a property of some compounds and complexes. Thus, polymorphs are distinct solids sharing the same molecular formula, yet each polymorph may have distinct physical properties. Therefore, a single compound may give rise to a variety of polymorphic forms where each form has different and distinct physical properties, such as different solubility profiles, different melting point temperatures and/or different x-ray diffraction peaks. Solvent medium and mode of crystallization play very important role in obtaining a new salt or a crystalline form over the other.
The discovery of novel solid forms, including amorphous forms and crystal forms, of pharmaceutically useful compound provides a new opportunity to improve the performance characteristics of a pharmaceutical product. Novel crystalline form L of (±)-desvenlafaxine benzoate has now been discovered.
Further, the present invention also relates to pharmaceutical composition of novel crystalline form L of desvenlafaxine benzoate and one or more pharmaceutically acceptable excipient and process for preparing them.